1. Field of the Invention
The present invention relates to the location of terminals, such as wireless mobile units, using distance measurements, such as satellite-based and/or wireless network-based distance measurements.
2. Description of the Related Art
Satellite-based navigational systems, such as the Global Positioning System (GPS), provide accurate position information to worldwide users. A satellite-based navigational system requires a terminal to be able to detect signals from at least three satellites in order to determine its location. When GPS signals from three satellites are received, a xe2x80x9ctwo-dimensionalxe2x80x9d position (e.g., latitude and longitude) of the terminal can be determined. When GPS signals are received from four or more satellites, a xe2x80x9cthree-dimensionalxe2x80x9d position (e.g., latitude, longitude, and altitude) can be determined. These preconditions, however, may not always be satisfied, especially when one or more of the direct satellite signals are obstructed, such as for a terminal located inside a building.
When the terminal is a mobile unit or other wireless communication device that is part of a wireless telecommunication network, its position may also be determined based on distance measurements available within the wireless network, such as the round-trip delay (RTD) and the pilot phase offset (PPO). The RTD is the time that it takes to transmit a signal from a cellular base station to the mobile unit and then back to the base station. The PPO is a measure of the code phase of a pilot signal as received by the mobile unit. If RTD and/or PPO measurements corresponding to three or more base stations are available, then the position of the mobile unit may be determined using wireless network-based position techniques including conventional triangulation schemes, such as time difference of arrival or angle of arrival.
One drawback for such wireless network-based positioning techniques is that the accuracy of the position determination is typically much lower than that provided by satellite-based positioning techniques. Another problem is that there may not always be three or more wireless network-based distance measurements available for the purpose of position calculation.
The present invention is directed to a technique that enables the determination of the position of a mobile unit in a wireless telecommunication network, where the mobile unit is configured with a global positioning system (GPS) receiver or other satellite-based navigational system receiver. According to implementations of the present invention, the position of the mobile unit is determined by integrating satellite-based measurements and wireless network-based measurements. As such, the present invention can be used to determine the position of the mobile unit when less than the required minimum number of satellite signals are available for conventional satellite-based positioning techniques. Moreover, the present invention can be used to provide more accurate positioning than would otherwise be provided by conventional wireless network-based positioning techniques.
Conventional schemes for integrating satellite-based distance measurements and wireless network-based distance measurements for determining the position of a mobile unit do not produce desirable results for several reasons. One problem is that the quality of the measurements, which come from different sources, are quite different. For example, errors in the round-trip delay (RTD) and/or pilot phase offset (PPO) measurements used in wireless network-based positioning techniques may be as much as ten times larger than the errors in the GPS pseudorange measurements used in satellite-based positioning techniques. GPS pseudorange measurement errors are typically about 10-25 meters, while typical RTD measurement errors are about 40-100 meters. Simply combining satellite-based distance measurements and wireless network-based distance measurements (e.g., in a conventional least mean squares (LMS) triangulation algorithm) without considering the corresponding measurement qualities may adversely affect the accuracy of the determined position.
Another problem with the conventional integration schemes is that no approach is adopted to reduce the influence of undesirable geometric distributions. Undesirable geometric distributions refer to certain orientations of the mobile unit with respect to the various sources of the distance measurements (i.e., satellites and/or base stations) that make accurate determination of the three-dimensional or even the two-dimensional position of the mobile unit difficult. For example, accurate three-dimensional position determination may be difficult when the mobile unit and three or more of the various sources of the distance measurements all lie in approximately the same plane. Similarly, accurate two- or three-dimensional position determination may be difficult when the mobile unit and two or more of the various sources of the distance measurements all lie in approximately the same line. In these situations, a position error known as the geometric dilution of precision (GDOP) arises. When such a situation exists, it is well known that errors in the distance measurements can be greatly magnified in a conventional LMS triangulation algorithm.
As a result of these problems, an integration scheme that simply applies a conventional LMS triangulation algorithm to a combination of satellite-based distance measurements and wireless network-based distance measurements to determine the position of a mobile unit will not typically provide satisfactory results.
The present invention is directed to a technique that effectively integrates satellite-based and wireless network-based distance measurements to provide accurate position determination for mobile units of a wireless telecommunications network. Embodiments of the present invention utilize one or both of the following technologies: (1) a weighting strategy that assigns an appropriate weight to each different distance measurement (e.g., different weights for satellite-based and wireless network-based distance measurements) based on a level of confidence in the accuracy of the distance measurement, and (2) a ridge regression strategy that reduces the influence of geometric dilution of precision on the mean square error of the calculated position. The present invention can be used to increase the accuracy of position determination for mobile units, as well as to remove the requirement of having to have all distance measurements come from a single type of source (e.g., all satellite-based distance measurements or all wireless network-based distance measurements) in order to determine mobile unit position.
In one embodiment, the present invention is a method for determining the location of a terminal, comprising the steps of: (a) assigning a weight value to at least one of a set of three or more distance measurements corresponding to three or more different sources relative to the terminal to generate a weighted set of distance measurements; (b) assigning a value for an insertion parameter; and (c) performing weighted ridge regression (WRR) processing based on the weighted set of distance measurements and the insertion parameter to determine a WRR estimate representing the location of the terminal.